"Servo-skulls form an important niche in Imperial work, serving as everything from auto-scribes that copy down important conversations and confessions of prisoners, to simple moving torches, hovering about their charge with candles and electric lanterns to illuminate the area."

and

"Servo-skulls are often used by Inquisitors and Inquisitor Lords as non-human assistants referred to as Familiars. In this case they are mentally linked to the Inquisitor through psychic or cybernetic means, allowing him to control them and see and hear through their senses."

are fine explainations of why do the humble members of the Inquisition use Servo-skulls. In fact - these 'familiars' are very important for whole Imperium of Man and "...to have one's skull chosen to serve as a Servo-skull is a great honour in the Imperium." for you can continue to serve the Emperor even after your death. It's like being put into Dreadnought coffin, but for the non-astartes folk!